1. The problem statement, all variables and given/known data An alpha particle is bombarded on 14 N. As a result a 17 O nucleus is formed, and a particle is emitted. This particle is a: (a) neutron (b) proton (c) electron (d) positron. The answer is given as (b). 2. Relevant equations The solution must take into account that the mass numbers have to balance on both sides of the equation. 3. The attempt at a solution (c) and (d) get ruled out because they do not lead to a balance in mass numbers. Since the balance would be possible for both options (a) and (b) I need some clues as to why only (b) is acceptable.